This is a study to assess the efficacy and safety of 3,4-diaminopyridine (DAP) in patients with Lambert-Eaton myasthenic syndrome (LEMS). LEMS is a rare disorder of neuromuscular transmission that produces weakness of varying severity and autonomic dysfunction. It is frequently associated with small cell lung cancer and results from an autoimmune reaction against the presynaptic nerve terminal. DAP is an orphan drug that has been used in other countries for over 20 years to treat patients with LEMS and other diseases with abnormal neuromuscular transmission, but which has not been approved for clinical use in this country. Treatments for LEMS include therapy for any underlying cancer, which frequently produces improvement in weakness as well; cholinesterase inhibitors, which usually produce only limited benefit; guanidine, which can produce severe side-effects; immunosuppression, which is usually only minimally or moderately effective; and plasmapheresis or high-dose immunoglobulin infusions, which produce only temporary improvement and are very expensive. Many patients with LEMS achieve marked improvement in symptoms and function with DAP. Experience from many years of use in other countries and from preliminary studies that we and others have carried out in the U.S. indicates that DAP is a safe and effective treatment for LEMS and can frequently make the difference between severe disability and relatively normal function in these patients. If DAP is approved for clinical use in LEMS, not only will it markedly improve the treatment of these patients but it will make DAP available for use in other disorders of neuromuscular transmission, such as congenital myasthenic syndromes, for which there are few therapeutic options. This is a double-blinded, placebo-controlled single-site, phase III study in which DAP or placebo capsules are administered for 5 days in hospital, during which time patients are monitored for clinical and electromyographic evidence of improved neuromuscular function, and for side-effects or toxicity. Upon completion of the blinded phase of the study, patients are given open- label DAP with pyridostigmine as long as there is demonstrable clinical benefit.